In the current production of solar cells, when an n-type or p-type impurity diffusion layer is formed in a semiconductor substrate, a method is used in which a diffusion source is formed on the substrate, and impurities are diffused into the semiconductor substrate by thermal diffusion. The diffusion source is formed by chemical vapor deposition (CVD) or solution coating of a liquid impurity-diffusing composition. When a liquid impurity-diffusing composition is used, for example, a thermally oxidized film is first formed on a semiconductor substrate surface, and then a resist having a predetermined pattern is laminated to the thermally oxidized film by photolithography. Using the resist as a mask, parts of the thermally oxidized film not masked by the resist are etched by an acid or alkali, and the resist is removed to form a mask of the thermally oxidized film. Subsequently, an n-type or p-type diffusion composition is applied to attach the diffusion composition to openings of the mask. Thereafter, impurity components in the composition are thermally diffused into the semiconductor substrate at 600 to 1250° C. to form an n-type or p-type impurity diffusion layer.